1. Field of the Invention
The invention relates to a control apparatus and control method for an automatic transmission mounted in a vehicle. More specifically, the invention relates to a control apparatus and control method for controlling a clutch-to-clutch shift.
2. Description of the Related Art
An automatic transmission mounted in a vehicle is constructed of a combination of a torque converter, into which output from an engine is input, and a gear shift mechanism which is driven by output from the torque converter. The automatic transmission is shifted automatically into a predetermined speed according to a request from the driver and the operating state of the vehicle by selective application and release of a plurality of friction engaging elements such as clutches and brakes so as to change the power transmission path of the gear shift mechanism. In this kind of automatic transmission, a friction engaging element for an engine brake is provided in addition to the friction engaging elements for shifting. Generally, when driving, this engine brake friction engaging element transmits power. When decelerating in a predetermined gear speed such as first range or second range, however, the engine brake friction engaging element is also applied, but in this case serves as an engine brake.
In this kind of automatic transmission, a shift may also be performed by switching friction engaging elements in such a way that control to apply one friction engaging element and control to release another friction engaging element are performed simultaneously (this kind of shift is often referred to as a clutch-to-clutch shift). In such a clutch-to-clutch shift, the timing of application of the one clutch is adjusted appropriately with respect to the timing of release of the other clutch, which results in good shifting characteristics (such as a good shift feeling for the driver).
In a clutch-to-clutch shift, a good shift feeling can only be achieved after first sufficiently tuning, for example, the degree to which the clutch to be released is released, the degree to which the clutch to be applied is applied, and the degree to which the engine torque is reduced.
JP(A) 10-184410 discloses a shift control apparatus for an automatic transmission which starts to reduce to the torque through engine control while simultaneously increasing the hydraulic pressure before the rotational speed on the input side of the automatic transmission starts to change, thus enabling the heat load and shock at the initial stage of engagement to be reduced. This shift control apparatus controls an automatic transmission that is provided with i) an automatic shift mechanism that outputs the rotation from the input side to the output side after changing the transmission path by releasing or applying a plurality of friction engaging elements, and ii) a hydraulic pressure circuit that switches the hydraulic pressure provided to a hydraulic pressure servo which releases or applies the friction engaging elements. This shift control apparatus includes i) an input rotational speed sensor that detects the rotational speed on the input side, ii) a pressure regulating circuit that regulates the hydraulic pressure to the hydraulic pressure servo, iii) an engine operating portion that adjusts the engine output torque, iv) a hydraulic pressure control circuit that issues to the pressure regulating circuit a command to increase the hydraulic pressure to the hydraulic pressure servo (hereinafter referred to as a “hydraulic pressure increase command”) of the friction engaging element to be applied before there is a change in the rotation on the input side of the automatic shift mechanism during a predetermined shift, and v) an engine control portion that issues a torque-down command (i.e., a command to reduce torque) to the engine operating portion simultaneously with the hydraulic pressure increase command.
According to the shift control apparatus disclosed in JP(A) 10-184410, as shown in FIG. 5, when a predetermined shift is made, a hydraulic pressure (application pressure) PA for a hydraulic pressure servo for the apply side friction engaging element is increased toward a target hydraulic pressure (PTA) for right before the input side rotation is changed. The target hydraulic pressure (PTA) is calculated according to, for example, input torque (TT). Further, the application pressure PA is then increased from the target hydraulic pressure by a hydraulic pressure change (δPTA) calculated based on, for example, a target rotation rate of change until the change in the input side rotation can be determined by detection with an input rotational speed sensor (i.e., until ΔN≧dNS). Before the change in the input side rotation is detected (i.e., while ΔN<dNS), the engine control portion outputs a torque-down command so that the engine torque is reduced at the same time the application pressure (PA) reaches the target hydraulic pressure (PTA) for example. By starting a torque reduction of the engine torque at the same time the application pressure is increased, the torque can start to be reduced before inertia torque is generated. As a result, fluctuation in the output shaft torque at the initial stage of engagement is able to be minimized, thereby reducing shock. Further, because the change in rotation is done not only using application pressure but also a torque reduction of the engine torque, the change in rotation can be produced with a low application pressure value, enabling the heat load on the friction members to be reduced during the initial stage of engagement.
The shift control apparatus disclosed in JP(A) 10-184410, however, also has the following problems. As shown in FIG. 5, the engine torque-down control starts at tSE+tTA, but after tTA, both the upward slope of the hydraulic pressure for the hydraulic pressure servo for the apply side friction engaging element (i.e., application pressure) PA and the downward slope of the hydraulic pressure for the hydraulic pressure servo for the release side friction engaging element (i.e., release pressure) PB become gentler. Moreover, after tTA, the engine torque also starts to gradually decrease. With this kind of design in which a plurality of control targets are made to gradually increase and decrease, the tuning of those control targets becomes difficult. It is therefore preferable to be able to obtain a good shift feeling without the need for this kind of difficult tuning.